1. Field of the Invention
The invention relates to testing of via chains, and more particularly, to a system and method for testing via chains quickly and efficiently by increasing the ambient temperature of the via chain.
2. Description of the Prior Art
Via chains are common structures in die packages, used to vertically link metal layers. When a via chain is placed under stress, electromigration (EM) occurs, whereby momentum from the ‘electron wind’ is passed to ions in the metallic lattice, and these ions are then transported to neighbouring materials. This constant migration will eventually result in vacancies collecting under the vias, which leads to failure of the structure. Therefore, each via chain has a particular lifetime associated with it. As modern semiconductor devices have densely packed interconnect structures, failure of via chains becomes a serious design issue. In order to predict when failure will occur, Black's equation is employed. Black's equation is shown below as equation (1).MTTF(Mean Time To Failure)=Aωj−neQ/κT)  (1)Where:A=a constantJ=current densityN=model parameterQ=activation energyκ=Boltzmann constantT=absolute temperatureω=metal wire width
As demonstrated by the above equation, EM measurements are very dependent upon temperature. One way of determining lifetime and EM measurements, therefore, is to stress the via chain using heat and electrical current. Joule heating is generated when electrical current is passed through the via chain. High Joule heating, however, complicates the data measurements by introducing other mechanisms into the system that are not related to die failure, and manufacturers tend to eliminate this method if possible. If only thermal heating is used, however, the maximum temperature that can be reached at wafer level is limited by the wafer chuck, which typically has a high temperature threshold of 150° C. At package level, the dies can be heated in ovens, but this process is again limited to a maximum temperature of about 200° C.; above this temperature, intermetallic formation will occur between Gold and Aluminium in the package structure.
It is therefore an objective in the field to find a via chain testing method that can place the via chain under stress at a high temperature ambience while limiting the amount of Joule heating that is used.